Wednesday 1 December 2010 13.16 EST
First published on Wednesday 1 December 2010 13.16 EST

The delicately balanced global climate talks in Cancún suffered a serious setback last night when Japan categorically stated its opposition to extending the Kyoto protocol – the binding international treaty that commits most of the world's richest countries to making emission cuts.

However the US congress refused to ratify it and remains outside the protocol.

The brief statement, made by Jun Arima, an official in the government's economics trade and industry department, in an open session, was the strongest yet made against the protocol by one of the largest emitters of greenhouse gases.

He said: "Japan will not inscribe its target under the Kyoto protocol on any conditions or under any circumstances."

The move came out of the blue for other delegations at the conference.

"For Japan to come out with a statement like that at the beginning of the talks is significant," said one British official. "The forthrightness of the statement took people by surprise."

If it proves to be a new, formal position rather than a negotiating tactic, it could provoke a walk-out by some developing countries and threaten a breakdown in the talks. Last night diplomats were urgently trying to clarify the position.

Japan gave no reasons for making its brief statement on the second day of the talks, but diplomats said last night that it represented a hardening of its line. "Japan has stated before that it wants only one legal instrument and that it would be unfair to continue the protocol," said one official who did not wish to be named.

Japan, which last night declined to clarify its position, has said in the past that it would not reject a new legally-binding overall agreement, but is concerned that it would be penalised if it signed up to cuts while other countries such as India and China were not legally bound to make similar cuts.

Britain and other countries, recognising the totemic significance of Kyoto for developing countries, have said that they would be prepared to agree to a second commitment period – as long as other countries also did so.

"This is a very bad start to the negotiations. The danger is that other countries may want to follow Japan's example and run away from binding commitments to cut emissions," said Poul Erik Laurisden, a spokesman with the aid agency Care International.

• This article was amended on 2nd December 2010. The copy originally stated that the Kyoto Protocol was adopted in 1992. It was in fact adopted in 1997. This has been corrected.